Automatic control systems are critical to all sectors of industry such as process control, discrete control, batch control (process and discrete combined), machine tool control, motion control, and robotics. One of the strongest needs in modern control systems is development and use of “open” and “interoperable” systems. Open, interoperable systems allow control devices made by different manufacturers to communicate and work together in the same system without the need for custom programming. “Fieldbus” is the common term used to describe these types of control systems.
The movement toward open, interoperable fieldbus systems is driven by device manufacturers and end users. Manufacturers want open, interoperable systems because it allows them to sell their products to more end users while reducing development costs. End users want open, interoperable systems so that they can select the best control devices for their system regardless of the device manufacturer.
There has also been a trend toward distribution of control functions into intelligent devices. In centralized control systems, a central controller performs all the control functions.
In distributed control systems, more than one control device operating in the system takes an active role in the control functions. Although both centralized and decentralized systems use a communication network, decentralized systems reduce overall system costs by reducing or eliminating the centralized controller functions between the control devices and the human-machine interface.
In order for distributed control systems to be truly open and interoperable, both the communications system and the user layer (above the communication system layers) must be specified and made open. An example of an open and interoperable distributed system is the fieldbus system provided by the Fieldbus Foundation. The FOUNDATION™ fieldbus user layer is described, e.g., in U.S. Pat. No. 6,424,872 entitled “BLOCK-ORIENTED CONTROL SYSTEM” (the “'872 patent”) and U.S. Pat. No. 7,272,457 entitled “FLEXIBLE FUNCTION BLOCKS” (the “'457 patent”), both assigned to assignee of the present application.
The 31.25 kilobits per second fieldbus (H1) and High Speed Ethernet fieldbus (HSE) used by the FOUNDATION™ fieldbus is described in part by International Electrotechnical Committee (IEC) Standard IEC 61158, the entirety of which is hereby incorporated by reference herein.
While the FOUNDATION™ fieldbus provides the open and interoperable solution for the H1 control capability, there is a great need to provide an open and interoperable solution for distributed control on a very high performance communication system typically called a fieldbus “backbone” network. The backbone network aggregates information from the lower speed control devices, e.g., the H1 and other control devices, which is used in supervisory and advanced control applications. The backbone is also needed for integration of control information into the enterprise's Management Information Systems (MIS).
One of the widely accepted standards for high performance communications signaling is Ethernet. Invented by Xerox in the 1970's, Ethernet has progressed from an initial speed of 10 Megabits per second, to 100 Megabits per second, to 1 Gigabit per second and beyond. Ethernet signaling is specified in an Institute of Electrical and Electronics Engineers (IEEE) standard (IEEE 802.3). Ethernet signaling is the underlying technology used by the Internet. The Internet protocols are specified by the Internet Engineering Task Force (IETF) and are issued as Request For Comment (RFC) specifications.
Although Ethernet/Internet technology provides the basic services for a high performance fieldbus backbone, it does not provide for all of the functions needed for use in distributed control systems. In particular, IEEE and IETF do not have suitable open and interoperable solutions for integration of distributed control systems (e.g., the H1 subsystem), system time synchronization, and fault tolerance.
The method of transferring information from lower speed fieldbuses to the Ethernet used by organizations such as Open DeviceNet™ Vendor Association, Inc., (“EtherNet/IP,”) and PROFIBUS International, (“PROFINet”) are not suitable for use in the high performance environment because they encapsulate the lower speed protocol packets in an Ethernet frame. This method, known as “tunneling,” is common in centralized control systems, but is inadequate for high performance distributed control systems. Although simpler to specify, tunneling would require too many Transport Control Protocol (TCP) connections with the resulting interrupt processing and memory overhead on the devices connected to the fieldbus backbone. In addition tunneling wastes much of the Ethernet bandwidth because the lower speed protocol packets (e.g., the H1 packets) are small and in many cases the Ethernet packet overhead would be bigger than a lower speed protocol packet.
Implementing a control system using wireless technology can provide many benefits. For example, utilizing wireless communication can further reduce user installation costs while facilitating connection to points physically or economically difficult to access. Wireless solutions allow easy access to additional measuring and actuation points for process supervision and control, process optimization, plant and personnel safety, and maintenance. Implementing a control system using wireless communication poses some challenges, however. One challenge, for example, is maintaining the openness and interoperability in the wireless environment in order to allow wireless field devices made by different manufacturers to communicate and work together in the same system. In typical applications where a specified communication protocol is utilized, for example, the one provided by the Fieldbus Foundation, the wireless field devices must be operable according to the specification. Another challenge is for the wireless field devices to be compatible with existing wired network connections and wired control devices.
Thus, what is needed is an open, interoperable solution optimized for integration of distributed control systems and other control devices in a high performance fieldbus backbone that includes communication and operability with wireless field devices.